Pacific Side

20 February 2015

Mapbox GL JS with Offline Vector Tiles on Cordova

Update:

Check out these repositories that do what is discussed in Solution 2 here:
https://github.com/trevorpowell/mapbox-gl-js-cordova-offline
https://github.com/trevorpowell/mapbox-gl-js-cordova-offline-example

Original Post:

Seen some questions about using Mapbox GL with offline vector tiles online as I've been looking into it for my own projects, and thought I might post some of what I've found. This is mainly for use cases where you might want an app for hiking, international use without data roaming, etc. There is a ticket open for offline support in the native version of Mapbox GL, but I wanted to see how it might work to use the JS version in a hybrid mobile app (Apache Cordova, AppGyver Steroids, etc.) since WebGL is supported in iOS and Android webviews now. I saw some discussion of this about two years ago with raster tiles generated in Mapbox Tilemill. My experience back then was that performance was slower when using a SQLite plugin in Cordova to load the tiles from the MBTiles database on demand, but it was better when I extracted the PNG files from the MBTiles database. Obviously, then you lose the benefits of MBTiles — deduplication of identical tiles and not having a ton of files to store. My hope was that with vector tiles being lighter weight, performance in a hybrid app might be better.

Mapbox Studio is to vector tiles what Tilemill was to raster tiles. Mapbox GL is another generation beyond Mapbox Studio — the vector tile sources are the same but the styling is different. A new style editor for Mapbox GL seems to be in the works, but for now there is a guide to follow. Mapbox Studio allows you to export vector tiles as an MBTiles database. Once exported, you can either serve the tiles from the MBTiles file or extract the tiles and serve them as PBF files. There is an example here on serving from an MBTiles file with Node.js that uses Mapbox's Tilelive. It was helpful in understanding some of how Mapbox GL and vector tiles work. Without a Node.js server in a hybrid mobile app, you would have to do things differently. Here are two possibilities on Cordova and some options for each.

Solution 1 – Extract vector tile PBF files from MBTiles database

To start with, use MBUtil to extract the vector tiles as PBF files. For example:

./mb-util --image_format=pbf test.mbtiles test

The files will be gzipped so without a way to add a gzip header when serving them, you could gunzip them:

gzip -d -r -S .pbf *

And re-add the PBF file extension:

find . -type f -exec mv '{}' '{}'.pbf \;

Appgyver Steroids runs a web server on the device that Mapbox GL JS can use when making XHR requests for the style JSON file (or you could just pass a JSON object to Mapbox GL JS for the style instead of a file reference) and the vector tile files. Plain Cordova does not have a web server, but you can use plugins like these to add one:

An alternative to using a web server plugin on Cordova might be to override the methods in Mapbox GL JS that make the calls for the style JSON and vector tiles and instead have them use the Cordova File plugin. Maybe in here:

Solution 2 – Serve vector tiles from MBTiles database using SQLite plugin

This would likely be the better solution; the only question is how it would perform in a hybrid app. The idea on Cordova is to use the SQLite plugin here to retrieve the tiles from the MBTiles database:
https://github.com/brodysoft/Cordova-SQLitePlugin

Again, this would require modifying the methods in Mapbox GL JS that load the vector tiles. You may have to gunzip the tile content then and could use Pako to do so:
https://github.com/nodeca/pako

Demos

I have played around with this some using the "touch" branch from the Mapbox GL JS repository:
https://github.com/mapbox/mapbox-gl-js/tree/touch

Here are some small demos on Cordova with very basic offline vector tiles using the Natural Earth coastline data:

23 March 2010

News Video Map

News Video Map

News Video Map

The most natural way of presenting world news content to me is on a map. Even without one presented to me, a geographical schema is my mind's automatic way of visualizing a list of world news articles or videos. My goal was to take a collection of world news video from providers like the AP, Reuters, and AFP, associate the content with geographic locations, and plot the results on a map.

This started as a one-day project I worked on for one of the Yahoo! Hack Days last year, and I've been playing with it since then when I have time. Hover over video clip thumbnails for a full second to open a preview of the associated video clip. Click on the play button in the preview to view the video.

The mashup combines data services and tools from the following Yahoo! providers:
Yahoo! News
Yahoo! Video
Yahoo! Maps
Yahoo! Placemaker
Yahoo! User Interface Library

It also relies on SWFObject for Flash embedding and this free image resizing proxy called Images.weserv.nl.

On the backend, there's a cron job that executes the PHP layer every hour. The PHP layer takes the Yahoo! News video feed for the World News category from the past seven days and runs all the video titles and descriptions through Yahoo! Placemaker to assign geographic locations to each one in latitudinal and longitudinal coordinates. The PHP layer then creates a JavaScript array of JSON objects from the data. For production use, it then rolls up the JSON data with the required JavaScript libraries and initialization script. The JavaScript layer creates the map and plots each video with a degree of randomization to prevent video clip thumbnails from stacking on top of one another.

The major issue to overcome in development was the amount of time required for data processing. Initially, the mashup fired an AJAX request to the PHP API that returned fresh data per request. It was taking over ten seconds for the page to load each request though, so I added a five minute cache to the API. The unfortunate user who hit a stale cache still had to wait over ten seconds for the page to load. In the end, I decided that since the purpose of the service is to provide a snapshot of videos from the previous week, it's not particularly important that the data be fresh to the minute. I switched to a cron-driven script that updates the JavaScript every hour, making it available in native JSON at page load rather than via a delayed AJAX request for XML. I also made a number of optimizations to the frontend — removing duplicate scripts loaded by the Yahoo! Maps API, rolling up all required JavaScript into one file, deleting data from memory when it's no longer needed, etc. Before putting this into full-scale production I would want to use an enterprise-grade image resizing proxy to scale the video clip thumbnails before they are loaded. As it is, it's faster to load the larger thumbnails directly from the cache server than to pass them through the free image resizing proxy.

Update: Have switched to enterprise-grade image resizing. It's more reliable than the free proxy and makes everything a lot faster. :)

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